In the automotive industry, there are various means and ongoing researches for improving vehicle efficiency such as engine thermal efficiency, vehicle mass, friction and pumping penalties, aerodynamics, brake/tire and gearbox losses as well as idling, lubricating, turbo charging and other technological challenges.
However, the most effective approaches are still today related to the improvements of the injection, combustion and after-treatment processes. The improvement of thermal efficiency of the combustion process directly and proportionally impacts on fuel efficiency and exhaust emissions. The fact is that the internal combustion process in an engine cylinder is impacted by numerous superimposed phenomena as illustrated in FIG. 1. All of these phenomena are tridimensional, time-dependent, with involvement of transient multi-phase reactive flows to be passively or actively controlled over a wide range of the engine operation. The diesel heterogeneous spray and following after-diffusion combustion are mainly controlled by timing and shaping of fuel discharged rates within ultra-short time fractions, which are currently close to a few hundreds of microseconds.
It is therefore acknowledge that one critical and viable solution for improving engine efficiency is directly related to the increased performance of the fuel injection equipment (in additional to the implementation of variable valve train).
There are a number of known approaches to perform combustion at highest thermal efficiency with complete combustion, as depicted in FIG. 2. One may be quite familiar with each of these approaches based on practical experiences how they impact on engine performance and emissions. For instance, in diesel engine design, it is standard: (i) to center the outlet of a fuel injector In a bowl and to center the bowl in the piston; (ii) to trade-off fuel injection pressure vs. air motion to provide required mixing; (iii) to supply sufficient air to meet peak torque limits; (iv) to trade-off injection timing and compression ratio for best fuel economy; and (v) to optimize fuel economy within emission constraints.
However, the entire diesel combustion process is still very complex, rapid and transient. The air-fuel mixture is extremely heterogeneous can vary in a wide range in terms of air/fuel charge (typically from about 4 to 20).
As diesel combustion is largely controlled by air-fuel mixing dynamics, an improvement of such dynamics could largely improve the engine efficiency.
From a more practical standpoint, in an effort to generate a fine spray with a quick break-up time, the most recent efforts have consisted In drastically increasing the injection pressure. Thus the pressure levels currently applied in automotive diesel injection equipment are very high (typically 1350-2400 bar for diesel injection, 50-100 bars for gasoline direct injection systems and 3-20 bars for gasoline manifold injection systems.
In this regard, it has been found that the fuel jet dynamics is characterized by the ratio between the jet kinetic energy based on pressure energy transfer and the capillary energy accumulated due to surface tension over the nozzle hole. The development of spray is occurred shortly after fuel exited the nozzle and it can be controlled if the Weber number We, which is proportional to the square of the jet velocity, is greater than about 40.
Accordingly current diesel injection equipment, where We reaches 105 to 106 (corresponding to injection pressure of 1600-2000 bars), allows to produce a good fuel droplet size (Sauter mean diameter—SMD) of about 25-40 μm in diameter, within a short breakup time brt (typically one microsecond or so).
In other words, increasing the fuel pressure allows to decrease the jet breakup time and to downsize the spray droplets.
However, increasing the fuel pressure has several drawbacks. First of all, injectors working with such high supply pressure need to have extremely narrow discharge lumens, and an increased number of such lumens compared to prior injectors. The injector manufacture thus becomes expensive.
In addition, the injection system requires adaptations in the pumping and cooling devices to be used effectively onboard to account for such high pressures. Overall, the extra energy needed for generating such increased pressure is significant, reaching approximately a few hundred Watts.
There are also maintenance issues due to the increase of injection pressure, and in particular a fatigue of the injector tip material(s) and an increased fuel temperature in the return lines.
There have been tentative solutions to improve fuel injectors in order to improve fuel spray generation.
In particular, US patent application 2002/0000483 A1, by Shoji et al. discloses a fuel injector nozzle in which fuel flow from a common source exits at the nozzle through separate concentric openings. The openings are at slightly different angles, such that the jets collide soon after exiting the nozzle. This collision is supposed to break the fuel jets into smaller particles quickly and uniformly.
However, the collision occurs at a relatively large distance from the jet outlets (typically more than 20 mm) and produces relatively large fuel droplets in the spray (more than 30 microns). Such known injection system therefore fails to generate a very fine fuel spray as close as possible to the injector outlets.
U.S. Pat. No. 6,272,840 B1 issued to Crocker et al. shows a gas turbine fuel injector in which fuel is injected into the combustion chamber through concentric rings. The pilot fuel injection ring and main fuel injection ring mix with air injected into the chamber through additional concentric injection rings. This injection system mixes the fuel and air more quickly and reduces the NOx emissions from the engine.
However, such known injector needs additional pressurized air assistance for generating the fuel spray, which would need additional components in the global injection system. In addition, such injector Is adapted for the steady state conditions of a turbine, and would not be applicable to the non-steady mode of operation of an internal combustion engine.
Finally, U.S. Pat. No. 5,771,866, issued to Staerzl discloses a nozzle for a low pressure fuel injection system in which two fuel conduits are associated in a coaxial and concentric relation with each other. The conduits have a common termination and are disposed within the open end of a cap. As fuel is caused to flow through the first conduit, air at atmospheric pressure is drawn into the second conduit. As the liquid fuel and the air reach the common termination of the conduits within the cap, the liquid fuel is atomized into a fine spray or mist. By providing a fine mist even at low engine speeds, the fuel injector nozzle does not require an air compressor.
Such air-assisted fuel injectors were intensively studied in mid- and late 90's without promising any improvement in droplet size and breakup timing needed for internal combustion engines, especially for diesel type applications.